1. Field of the Invention
This invention relates to self-lubricated rock bits.
More particularly, this invention relates to a self-lubricated rock bit with a means to relieve an overpressure condition within the rock bit with negligible loss of lubricant.
2. Description of the Prior Art
There are several patents that deal with a lubricant relief valve for rock bits. Most representative of these patents is U.S. Pat. No. 3,476,195 which discloses a relief valve to relieve overpressure conditions within a bit. This invention resulted from the drilling of deep wells with lubricated rock bits equipped with lubricant seals and also provided with flexible pressure compensators and vent holes to equalize the pressure between the lubricant within the bearing and the drilling fluid or mud surrounding the bit. Excessive pressure or heat buildup within the bits caused the pressure within the bit to exceed the drilling fluid pressure. This internal overpressure condition caused the flexible compensator to be extruded into a vent hole which often resulted in the destruction of the compensator and hence, loss of lubricant which extrudes from the drilling bit into the borehole.
It has been determined that the basic cause of the trouble is a pressure buildup within the bit brought about by cracking and thermal expansion of the lubricant. In deep borehole wells the bit is often subjected to very high pressures and temperatures, particularly in the vicinity of the journal bearings.
This invention relieves this overpressure condition by providing a valve, similar to a conventional pneumatic tire valve, that serves to relieve the pressure adjacent the compensator before it can rupture the compensator. These one-way valves are low pressure in nature and permit outflow from the lubricant chamber to the outside of the bit while blocking any reverse flow of contaminating fluids through the valve.
This invention is disadvantaged in that the low pressure relief valve is physically located adjacent the compensator diaphragm some distance away from the journal bearing of the rock bit. Since the excessive heat which causes the cracking of the lubricant and the high pressure condition is generated on the bearing surfaces between the journal and the cone associated therewith, there is danger of journal bearing seal failure. The seal is located between the cone and the journal bearing and the pressure buildup is immediately adjacent the seal. Hence, locating the overpressure valve some distance away from the initiation of this high pressure source increases the likelihood of seal failure due to the overpressure condition, displacement of the lubricant from the bearing and further contamination of the lubrication system.
The present invention provides a means to relieve the gases resultant from the cracked lubricant within the journal bearing at the source. In other words, a cavity is provided adjacent the O-ring seal so that excessive pressure within the bearing forces the O-ring locally out of the seal gland and into the cavity, thereby relieving the source of pressure past the seal that is displaced within the relatively small cavity in the seal gland boss or flange surface. Therefore, excessive pressure is relieved at the source of the problem in the rock bit.
Relieving the pressure adjacent the journal bearing through a displaced O-ring seal prevents further contamination of the lubrication system with gases and debris.